Photonic (also called optical) cross-connects are based on the idea that a channel (more exactly a single signal) from a plurality of received WDM signals (wavelength division multiplex signals) can be redirected into any one of transmitted WDM signals. Most of the cross-connects avoid wavelength conversion, because it is cost effective and, if optical-electrical conversion and 3R-regeneration is not used, reduces the signal quality. Another restriction for the signal quality comes from the wavelength selective elements. These problems increase, if DWDM signals (dense wavelength division multiplex signals) are transmitted.
To singularize single channels (single signals) of a DWDM signal and to combine a plurality of channels to a DWDM signal, it is necessary to use very narrow filters or similar wavelength selecting elements. This results in a reduction of signal quality. For dropping and adding channels (single signals) from and respectively to a DWDM signal, wavelength selecting elements are also necessary, but neither the quality of the dropped channels nor the quality of the added channels should be reduced. Beside this, a certain switching flexibility is also necessary for outputting drop signals and for inserting add signals.
Modern photonic cross-connects comprise optical multiplexers, demultiplexers, tuneable filters and wavelength selective switches. A photonic cross-connect is disclosed in the patent application US 2006/0098981 A1. Each through or express channel has to pass through a WSS (wavelength selective switch) and a multiplexer. This can be accepted in most cases. But each add and each drop signal has to pass through three narrow filter elements, this means that each signal (referred to as channels) has to pass three narrow filter elements in an add path and at least three narrow filter elements in an drop path even in a short point to point connection. The result is a significant reduction of signal quality.
WS-switches are used for realizing cross-connect and add-drop functions. A WSS functionality can be realized by different technologies. By means of these technologies, e.g. micro-electro-mechanical-systems (MEMS), an optical WDM signal received by an input port can be switched frequency selective to a plurality of output ports and vice versa. The realisation of a frequency-selective switching matrix with a plurality of inputs and outputs is possible by applying these elements.